Modified and unmodified starch products are extensively used for a variety of non-food and industrial applications. They have, traditionally, been used to size or finish textiles and papers, as adhesives (e.g., corrugated and laminated paper boards, remoistening gums, wallpapers, etc.), flocculants, binders (e.g., foundry core binders), fabric printing aides, thickeners and many other divergent non-food and industrial applications.
In the coated and shaped article manufacture, the trade presently relies upon synthetic polymeric materials which are primarily produced from petrochemical raw materials. Petrochemicals are a depletable natural resource. Within recent years, world-wide demand for petroleum-based products has adversely affected the cost and availability of synthetic polymers. Starches are readily available and replenishable with each crop year. Potential starch product usage would substantially increase, if it were possible to alter or correct certain inherent defects which have heretofore rendered starch products unsuitable for coating and/or shaped article applications.
Starches are inherently unstable against physical, chemical, bacterial and enzymatic degradation. Starches vary in amylopectin and amylose content. Waxy starches consist essentially of amylopectin with only trace amounts of amylose. Corn starch and other conventional starches such as tapioca, potato, wheat typically contain 16-24% amylose (dry solids weight basis) with the balance thereof being amylopectin. Amylose fractions are comprised almost exclusively of amylose while certain high amylose hybrid corn starches have an amylose content of about 40-70%.
Both high and low amylose starches have been used to coat substrates. The low amylose starches usually disperse readily into aqueous systems to provide an acceptable coating vehicle. Unfortunately low amylose starch coatings are prone to swell and readily disperse in water. In contrast, the high amylose starch coatings are typically insensitive towards water and possess adequate structural strength. Unfortunately, high amylose starches cannot be effectively converted into a uniform starch paste and directly utilized under ambient conditions to coat substrates. Both the high amylose starch and the low amylose starch coating techniques typically involve physical manipulation of starch molecules without altering or modifying the inherent starch compositional defects.
In U.S. Pat. No. 3,696,072 by G. A. Reynolds et al. there is disclosed hydrophobic, diethylenically unsaturated hydroxyl or amine containing polymers. By dispersing these diethylenically unsaturated polymers into organic solvent systems along with certain photoinitiators, ultraviolet curable coatings may be obtained. U.S. Pat. No. 3,936,428 by Rosenkranz et al. similarly report hydrophobic, photopolymerizable, N-methylol polyol polymeric compositions.
Numerous starch polymerizates have been reported. Caldwell et al. in U.S. Pat. No. 2,668,156 discloses that certain water-dispersible, ethylenically unsaturated starches will undergo homopolymerization in the presence of polymerization initiators. These starch homopolymerizates reportedly have reduced water-dispersibility properties and are useful for many conventional starch applications such as adhesives, sizing agents for textiles and papers, etc. The Caldwell et al. polymerizable starch system apparently produces a uniform homopolymerizate. Apparently a substantial portion of the starch ethylenic unsaturation either remains unpolymerized or undesirably intrapolymerized since the homopolymerization thereof becomes difficult at starch ethylenic unsaturation D.S. levels of 0.07 or higher.
Graft starch copolymers are also known (e.g., see U.S. Pat. Nos. 3,061,471, 3,061,472 and 3,095,391). Brockway et al. disclose granular starch polymethacrylate grafts by polymerizing granular starches in the presence of methyl methacrylate monomers, activators and initiators to provide a non-water-dispersible starch product (e.g., see Journal of Polymer Science, Part A, Vol 1, pages 1025-1039, 1963). Similarly, grafting by copolymerizing styrene and allyl starches (e.g., see Makromol Vol. 18-19, page 322, 1956), butadiene, styrene and acrylontrile (e.g., see Canadian Pat. No. 549,110 by Borunsky), acrylonitrile, acrylamide and acrylic acid (e.g., see C. A. Wilham et al. Polymerization Studies with Allyl Starch, Journal of Applied Polymer Science, Vol. 7, pages 1403-1410, - 1963) have been reported. The grafted non-starch polymer chains become an integral and unextractable part of the composite starch polymerizate and increases its hydrophobicity.
At one time allyl starches appeared potentially useful as starchbased coatings (e.g., see J. P. Radley, Starch and Its Derivatives, 4th Ed., 1968). Unfortunately, the allyl starch coating systems are plagued with difficulties such as non-homgeneity, brittleness, inflexibility, poor water-resistance and limited solubility in organo solvent systems (e.g., see Wilham et al. article cited above).
The art has long sought a polymerizable starch composition suitable for use as a protective coating and which will produce a starch polymerizate having improved resistance towards physical, chemical, bacteriological and enzymatic degradation. A water-dispersible and copolymerizable starch composition which could be applied to a substrate via an aqueous vehicle and thereafter copolymerized to a starch coating possessing excellent tensile strength and elongation, flexibility, impact and dynamic peel strength, water-and detergent-resistant properties would fulfill a long-felt need.